Let's view an example:
Teacher Agnes is ill. So 4 periods must be substituted. (see picture 1a). (Pictures are only visible if you are logged in as a user of this forum. Please click on the pictures to enlarge them.)
Students always have 6 hours per day and students gaps are not allowed.
But it's possible to move lessons forward at period 5.
You can see teachers which can substitute in the bottom of picture 1b.
You can see possible move forward periods at the top of picture 1b.
As you can see teacher Max teach the same group at the same day during the last period. So he can move forward his lesson. Also longer chains over several days are visible here.
If you select a move forward from the top TiTiTo will add comments into the substitution table (see picture 1c).